Rubber modified polymers prepared from monovinylidene aromatic monomers such as styrene and unsaturated nitrile polymers such as acrylonitrile have gained commercial utility as tough engineering plastics. Such plastics find widespread use in household appliances, automotive and architectural applications.
Government codes are requiring that polymers for the above applications have improved flame-retardant properties. The methods used to achieve the desired levels of fire retardancy has, in many cases, caused a deterioration of other physical properties such as toughness and impact strength.
Flame-retardant materials such as the halogenated aliphatic and aromatic compounds which are compatible with the styrene polymer family of plastics often act as plasticizers and lower the modulus of the polymer causing a reduction in the tensile strength of articles molded from the polymers.
Certain metal oxide compounds, e.g., antimony oxide and combinations of halogenated organic compounds provide a synergistic fire retardant effect in many polymer systems. However, such combinations are not predictable in that many such combinations lower the melting point of the polymer causing it to pyrolyze more readily, hence, actually increasing flammability. Beyond flammability such combinations have caused the polymer systems to degrade during heat processing or on exposure to light.
Polymeric halogenated plasticizers such as polyvinyl chloride and chlorinated olefins have been tried to overcome the deficiencies of the lower molecular weight halogenated organic compounds. Such polymeric materials, however, as used in the styrene family of plastics have lowered their heat stability during processing and given lower physical properties particularly lower modulus, heat distortion and impact strength.
A need exists in the art for styrene polymers and copolymers which exhibit improved fire retardant properties without a corresponding decrease in other physical properties such as modulus, tensile and impact strength.